It is known for supervisory systems of point-to-point optical networks to use Optical Time Domain Reflectometer (OTDR) measurements to monitor optical links, specifically each joint (splices or connectors) in each link, to locate a fault when it occurs.
In point-to-multipoint optical networks, such as Passive Optical Networks (PON), however, respective ends of the network links are connected in common at the single point at or adjacent a network element (OLT) and, when in service, the opposite ends of the plurality of links (i.e., the multipoint ports) are each connected to a respective one of a plurality of optical network units (ONU). Consequently, classical OTDR measurements cannot be made from the common or single point to unambiguously characterize all or part of each link individually because the backreflected light from the different links will be summed together when detected at the common (single) point.
It has been known for some time to address this limitation by placing a reflective optical element at a demarcation point, this demarcation point being either at the customer end of the network link, or at some intermediate point “downstream” from the OLT beyond which the network service provider is not responsible for the fiber link to the customer ONU. The reflective element is generally highly reflective at an out-of-band wavelength reserved for testing (e.g. in the U band) and highly transmissive in the wavelength bands normally used for data-carrying optical signals. (See, for instance: Enomoto et al., “Over 31.5-dB dynamic range optical fiber line testing system with optical fiber fault isolation function for 32-branched PON”, proceedings of Optical Fiber Communications Conference 2003, paper ThAA3; Koshikiya et al., “Newly developed optical fiber line testing system employing bi-directional OTDRs for PON and in-service line testing criteria”, IEICE Transactions on Communications, Vol. E90-B, No. 10, October 2007 pp 2793-2802).
When short light pulses at this out-of-band testing wavelength are launched into the common point by the OTDR, each highly-reflective element produces a corresponding discernable “localized event”, specifically a peak, in the resulting OTDR trace. The peak allows the position of the highly-reflective element to be determined, in terms of its optical distance from the common point, and, hence, the associated link to be identified based on predetermined knowledge (e.g. obtained during initial network commissioning, or during subsequent network expansion). Using this approach, the supervisory system can, in principle, qualitatively or quantitatively observe degradations in each optical path extending between the OTDR and the highly-reflective element by comparing the actual measurement of the peak to an initial baseline or reference level.
For convenience of description, hereinafter this technique of using reflective elements installed at such points along optical paths in the network will be described as “High Reflectance Demarcation” (HRD).
It has been proposed to measure the absolute loss of multipoint links in a passive optical network from the launching point of the OTDR to reflectors having known reflectance values at the end points of the multipoint links, using a reference reflectance of known value at the OTDR launch side. (See N. Gagnon (http://users.encs.concordia.ca/˜bjaumard/Conferences_and_Seminars/OON_Workshops/OON—2007/Slides_OON—2007/OON—2007_EXFO_Gagnon.pdf, slides 15-17). Such an approach also bears similarity to that used in the IQS-12001B test system manufactured by EXFO Electro-Optical Engineering Inc. for testing fiber patchcords, as illustrated in FIG. 4 of EXFO Application Note 161. However, in the case of the IQS-12001B measurement procedure, it is the patchcord loss that is assumed known, thereby allowing the end reflectance to be calculated.
An OTDR means, for instance comprising a reference reflectance, suitable for use in such HRD applications, is described in U.S. Pat. No. 5,754,284 (Leblanc et al.), which is commonly owned with the present invention. More generally, however, any OTDR means comprising normalization means to measure the ratio of the detected backreflected pulse power to the launched pulsed power can be used for such measurements.
A drawback of the afore-described prior art methods is that the measured OTDR traces may also comprise localized events that arise from reflective events such as high splitter return loss, connectors, etc., in one or more of the optical paths that is/are not common with all the other optical paths (i.e. not in the F1 cable of a PON network, for instance). In general, the amplitudes of these strong localized events are approximately wavelength-independent. In the case where the point-to-multipoint network comprises two or more stages of splitting, a wavelength-independent reflective event before a second splitter may cause a localized event having an amplitude similar to that of a nearly 100% reflectance beyond the second splitter. It may be difficult to identify initially which localized reflective events are from reflective elements installed for HRD purposes and which localized reflective events are from other network reflective events. Initial characterization of the point-to-multipoint network before commissioning may then be difficult, and the measurement of optical path loss in a particular multipoint link may become unreliable.
It is desirable for the reflective element used for HRD to be highly-reflective, typically returning almost 100 percent of incident power, so as to improve peak detection accuracy and extend measurement range. A high reflectance (i) is more stable with respect to environmental changes, (ii) is generally easier to produce with tighter nominal reflectance tolerances, and (iii) reduces the probability that another localized event, such as a reflection from a wavelength-independent reflective artifact as mentioned hereinbefore, will have an amplitude similar to that generated by the reflective element. Unfortunately, however, the relatively large reflections from such a highly-reflective element may detract from OTDR measurements, such as the measurement of Rayleigh backscattering (RBS) of the optical path. (It should be noted that, for the case when one of the multipoint links has been identified as exhibiting a strong loss, e.g. due to a fiber break, etc., an RBS measurement can be very useful in determining the location of that loss, despite the presence of superposed RBS from the other non-damaged multipoint fibers in the OTDR traces. This is particularly true where the point-to-multipoint network comprises two levels of splitting and the break occurs after the first splitter “downstream” from the OLT.) More particularly, large OTDR peaks corresponding to strong reflections may be followed by long dead zones due to detector recovery time and “undershoots” if the receiver is only marginally stable, which limits OTDR spatial resolution. This reduces fiber fault localization capabilities of the system since what is shown on the OTDR trace is not really the fiber RBS signature. In addition, the necessarily strong attenuation of the portion of the OTDR pulses transmitted through this highly-reflective element will render the detection of RBS downstream from the HRD impractical.
Araki et al (“High spatial resolution PON measurement using an OTDR enhanced with a dead-zone-free signal analysis method”, Symposium on Fiber Optics (SOFM04), Boulder Colo., September 2004, pp. 69-72) describe a method to overcome this dead zone problem, using an additional OTDR measurement taken “upstream” from the ONU. Also, in an article entitled “Newly Developed Optical Fiber Line testing System Employing Bi-Directional OTDRs for PON and In-Service Line testing Criteria”, IEICE Trans. Commun., Vol. E90-B, No. 10 Oct. 2007, Koshikiya et al. describe a method of locating faults in each of the PON links by means of OTDRs operating bidirectionally each at a different wavelength, a common OTDR at the central office and an additional OTDR at each customer premises. Unfortunately, although these methods might be effective, they would be unduly costly for most commercial applications.